1. Field of the Invention
The present invention relates to an actuator for joints of a manipulation arm, and more particularly to a magnetic-controlled actuator with an auto-locking function for joints of a manipulation arm.
2. Description of Prior Art
In 1984, the International Organization for Standardization gave a definition of robot in ISO 8373, which defined a robot as “an automatically controlled, reprogrammable, multipurpose, manipulator programmable in three or more axes, which may be either fixed in place or mobile for use in industrial automation applications.” In 1994, an industrial robot is officially defined as an automatically controlled, reprogrammable, multipurpose manipulator programmable in three or more axes. The robot should include the manipulator, actuator, and control system of software and hardware. In general, an industrial robot has a manipulator and a memory device, and the latter could be a variable-sequence control device or fixed-sequence control device. The robot is controlled to move, rotate, or expand and contract by signals sent from the memory device.
The research on the core technology of the robot is significantly developed in the developed countries, such as Europe, America, and Japan. The motor is mostly used to the drive mechanism for the traditional humanoid robot. At present, three types of manners are mainly introduced to drive the humanoid robot: a stepping motor, a speed reducing gear motor, and a high-torque brushless motor.
The stepping motor can generally be categorized into three types according to structure: permanent magnet (PM) stepping motor, variable reluctance (VR) stepping motor, and hybrid stepping motor. The rotor of the PM stepping motor is made of the permanent magnet. The rotor can produce holding torque due to the inherent magnetic, even the winding is not excited. In addition, the rotor of the VR stepping motor is made of processing the high permeability material. The rotor can not produce holding torque when the winding is not excited due to the absence of the magnetic force produced from the stator winding. Because the rotor can be particularly designed to increase efficiency, the VR stepping motor could provide larger torque. In general, the stepping angle of the VR stepping motor is 15 degrees, and the VR stepping motor is usually provided for machine tools which can provide larger torque and accurate positioning. In addition, the rotor of the hybrid stepping motor is made of installing a number of gear-shaped protruding electrodes and installing axial-direction permanent magnets. Hence, the hybrid stepping motor can be regarded as the combination of the PM stepping motor and the VR stepping motor. Accordingly, the hybrid stepping motor has the advantages of both the PM stepping motor and the VR stepping motor, namely, including the advantages of high accuracy and high torque. In general, the stepping angle of the hybrid stepping motor is smaller than the above-mentioned stepping motors and is between 1.8 degrees and 3.6 degrees. The stepping motor has the following features and advantages:
(1) The simple system structure; (2) The rotating speed is proportional to the frequency of the digital pulse; (3) The stepping motor is easily controlled; (4) The position sensor is not needed; (5) The lower costs; (6) The stepping motor is easily integrated with computers or digital machines; (7) The carbon brushes and the slip rings are not needed; (8) The high reliability; and (9) The use life is limited by the bearing.
The stepping motor, however, has the following disadvantages:
(1) The efficiency is lower; (2) The out-of-step condition easily occurs when the stepping motor operates at a high-speed or high-torque condition; (3) The resonance phenomenon is easily produced at a specific frequency; and (4) The less reliability is performed in the heavy-load condition.
The speed reducing gear motor is most commonly used in the robot application. In general, the speed reducer must have the very low backlash when operating in very high axial- and radial-directional loads, thus converting a high input speed into a low output speed and sending a large output torque. The speed reducer is easily installed and accurately aligned on the motor so that the generated noise and vibration are least. The speed reducing gear motor has the following features and advantages:
(1) The scheme is used in low-speed high-torque applications; (2) The motor can be easily designed and controlled for using in high-efficiency operation; (3) The simple feedback control can be implemented; (4) The speed reducing gear motor is easily integrated with computers or digital machines; (5) The high reliability; and (6) The use life is limited by the bearing.
The speed reducing gear motor, however, has the following disadvantages:
(1) The miniaturization is not easily completed; (2) The position ability is limited by the speed reducing mechanism; (3) The mechanism is often maintained; (4) The system structure is complicated; (5) The costs of the speed reducer is higher; and (6) The key technology has been mastered by foreign companies.
NdFeB magnet is used widely in brushless DC motor. Compared with other kinds of motors, the brushless motors has certain advantages, such as small volume, high power, and high torque, and also lower EMI and easy maintenance due to the absence of the carbon brush. In the high-torque robot application, however, the above-mentioned scheme has certain disadvantages. For example, a large-volume motor is required, an overheating issue occurs due to the over current, and the motor has a low efficiency.
Accordingly, it is desirable to provide a magnetic-controlled actuator with an auto-locking function for joints for a manipulation arm that produces a high cogging torque due to interaction of the permanent magnets, thus providing a sufficient locking force to lock the non-stationary inner-layer mover when a power failure occurs.